Lost cameras after NVR firmware update

Updated NVR firmware to the latest and lost all my cameras (empty ports). Rest one camera and got it back. No luck with remaining 3 cameras.
Tried the working camera on all ports and is working fine. It seems that the issue is with the cameras. Am I resetting the cameras correctly?
What is the sequence? Thanks for any input.

After several frustrating attempts I managed to reset all the cameras. The steps described in documentation did not work. It seems that holding the camera reset button for 10 sec is not enough. Finally everything is back and operational.

Need to hold the reset button for 15-20 seconds. Resetting the camera is the best thing to do because of the password that NVR trying to use to communicate with the camera.

Thank you very much for your response.
That was my understanding. Previous software allowed me to change password between camera and NVR. After the NVR firmware was updated, NVR stop connecting to cameras.
Only resetting cameras could help. Holding the reset button for about 20 seconds did the trick.
Thanks again for your input.
Paul ​